The regulations applicable in certain Scandinavian countries allow the use of tires that have studs to improve the grip of the tires on frozen and particularly slippery surfaces. These studs are positioned in housings made on the tread blocks situated on the tread.
The blocks of rubber that make up the tread of these tires thus, as a general rule, include numerous transverse incisions in order to improve the grip of the tire on snowy surfaces.
The manufacture of these particular types of tire involves a first step that culminates in the production of a vulcanized tire in which the blocks have incisions and housings intended to accept the studs, and a second step during which a suitable method is used to fit the studs into the said housings.
In order not to degrade the firmness with which the head of the studs is held in its housing, the incisions are interrupted within a region, generally a circular region, lying around the housing intended to accept the stud.
To do this, the mould intended for producing these tires suited to severe winter conditions comprises lamella intended to form the incisions, and pins intended to form the housings capable of accepting the said studs.
The lamella consist of elements in the form of thin and rigid, generally metal, partitions of a height that corresponds to the depth of the incision, and of a length that corresponds to the length of the incision. These lamella may have a wavy, straight or an even more complicated shape depending on the desired type of use. The lamella are positioned projecting in the part of the mould intended to give the tread its definitive shape, and which forms the mould lining.
The pins have the overall shape of that part of the stud which is inserted in the housing, and are embedded in the block of rubber at a predetermined point dependent on the noise and grip criteria defined by the tire designer.
If the shape of the former used to mould a tread block and as illustrated in FIGS. 1 and 2 is examined more closely, it may be seen that the lamella are interrupted around the region situated a given distance away from the centre of the pin. The lamella L1 and L6 run the entire width of the tread block. By contrast, the lamella in rows 2, 3, 4 and 5 are formed of two aligned lamella positioned one on each side of a circle surrounding the base of the pin P. The row 2 lamella is broken down into L21 and L22, the row 3 lamella is broken down into L31 and L32, the row 4 lamella is broken down into L41 and L42 and the row 5 lamella is broken down into L51 and L52. In addition, it can be seen that lamella L21, L22, L31, L32, L41, L42; L51 and L52 have different lengths according to the position they occupy about the space formed by the circle surrounding the pin P as can be seen in FIG. 2, which is a view from above of the lining element G shown in FIG. 1.
It then follows that the manufacture of a lining of this type entails the production of a great many different lamella the lengths of which are tailored to suit the positions that the pins occupy on the tread block element
It will be noted too that the pin P may also be situated at very varying points across the tread block as is shown in FIG. 3. On the basis of this example it is found that the lamella element L31 in FIG. 3 is far shorter than the lamella element L31 illustrated in FIG. 2. That increases the variety of lamella that have to be produced and fitted into the mould lining accordingly.
Finally, determining the final position that the pin will occupy on the block of rubber may be done at a late stage in the development of the tire because of the numerous optimization loops that are needed when seeking out the best grip/noise compromise.
All of the foregoing means that this approach has a serious impact on the cost of such a lining.